1. Field of the Invention
The present invention relates to a temperature sensors; and in particular to processing a temperature sensor output signal that varies non-linearly with temperature.
2. Description of the Related Art
Many internal combustion engines are now controlled by a sophisticated microcomputer based system. The microcomputer responds to different parameters of the engine which are sensed. One of those parameters is engine temperature that is derived from a sensor sensing the temperature of the coolant for the engine. A common temperature sensor produces an output signal that is converted to a pulse width modulated signal indicating the sensed temperature.
A characteristic of the PWM output signal, such as the duration of either the high or low signal level portion, varies to denote the temperature that was sensed. In many sensors of this type, the signal varies in a non-linear manner with temperature. On a graph of the PWM signal values versus temperature, the resultant plot would be a series of linear sections having different slopes connected together.
Not only is the relationship between the PWM signal values and temperature characterized by sections with different slopes, those slopes change for different starting temperatures of the engine. As a result, the control circuit has to employ relatively complex signal processing to accurately convert a value derived from the PWM sensor signal to the corresponding temperature. A common prior technique involved storing, in the engine control computer, sets of data defining the slopes of each linear section of the PWM sensor signal to temperature relationship for different starting temperatures. The accuracy of the signal conversion depended upon the number of different slope segments employed to represent each starting temperature based signal relationship.
When the engine started, the initial temperature was sensed and used to select the set of data for the associated PWM sensor signal to temperature relationship. Each time thereafter that PWM sensor signal was read by the engine computer, the slope for the corresponding section of the selected set of data was determined and used to convert the PWM signal value into a temperature value. That technique required several data access and calculation steps and therefore required a substantial amount of computer processing.
Thus there is a desire to provide a simpler method for processing the PWM signal values to produce the corresponding temperature for use in the engine control circuitry.